Led-based light sources for light emitting devices and lighting arrangements with photoluminescence wavelength conversion

ABSTRACT

An LED-based light source for generating light having a selected dominant wavelength λ ds  comprises a package housing a plurality of LEDs consisting of LEDs from first and second wavelength bins. The first wavelength bin comprises LEDs having a dominant wavelength λ d1  that is within a first wavelength range and the second wavelength bin comprises LEDs having a dominant wavelength λ d2  that is within a second wavelength range. The first wavelength bin can comprise LEDs having a dominant wavelength that is shorter than the selected dominant wavelength whilst the second wavelength bin comprises LEDs having a dominant wavelength that is longer than the selected dominant wavelength. The wavelength bins and number of LEDs are selected such that in operation the dominant wavelength of the combined light emitted by the source is the selected dominant wavelength. Lighting arrangements and light emitting devices incorporating such light sources are disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 61/475,134, filed Apr. 13, 2011, entitled “Lightemitting devices with remote phosphor wavelength conversion componentand LED-based light sources therefor”, by Li et al., the specificationand drawings of which are incorporated in their entirety herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to lighting arrangements and light emittingdevices that use LED-based (Light Emitting Diode-based) light sources toexcite a photoluminescence material, typically a phosphor, to generate adesired color of light. In particular, although not exclusively, theinvention concerns lighting arrangements that use a photoluminescencewavelength conversion component that is positioned remotely to the lightsource. More particularly the invention concerns LED-based light sourcesfor use in such lighting arrangements and devices.

2. Description of the Related Art

White light emitting LEDs (“white LEDs”) are known and are a relativelyrecent innovation. It was not until LEDs emitting in theblue/ultraviolet part of the electromagnetic spectrum were developedthat it became practical to develop white light sources based on LEDs.As taught, for example in U.S. Pat. No. 5,998,925, white LEDs includeone or more phosphor materials, that is photoluminescence materials,which absorb a portion of the radiation emitted by the LED and re-emitlight of a different color (wavelength). Typically, the LED chip or diegenerates blue light and the phosphor(s) absorbs a percentage of theblue light and re-emits yellow light or a combination of green and redlight, green and yellow light, green and orange or yellow and red light.The portion of the blue light generated by the LED that is not absorbedby the phosphor material combined with the light emitted by the phosphorprovides light which appears to the eye as being nearly white in color.

Due to their long operating life expectancy (>50,000 hours) and highluminous efficacy (70 lumens per watt and higher) high brightness whiteLEDs are increasingly being used to replace conventional fluorescent,compact fluorescent and incandescent light sources.

Typically in white LEDs the phosphor material is mixed with a lighttransmissive material such as a silicone or epoxy material and themixture applied directly to the light emitting surface of the LED die.It is also known to provide the phosphor material as a layer on, orincorporate the phosphor material within, an optical component (aphotoluminescence wavelength conversion component) that is locatedremotely to the LED die(s). Advantages of providing the phosphor remoteto the excitation source are a reduced likelihood of thermal degradationof the phosphor material and a more consistent color of generated light.

The inventors have discovered that the dominant wavelength of theexcitation light (typically blue) used to excite the phosphor can have asignificant effect on the color and/or color temperature of the lightemitted by the arrangement/device. For example for a 3000K white lightemitting arrangement a variation of 2.5 nm in the excitation wavelengthresults in a color shift of about one MacAdam ellipse in the output ofthe arrangement. The present invention arose in an endeavor to providean LED-based light source for use with a photoluminescence wavelengthconversion component that at least in part overcomes the limitations ofthe known sources.

SUMMARY OF THE INVENTION

Embodiments of the invention concern lighting arrangements and lightemitting devices comprising LED-based light sources that are operable togenerate excitation light (typically blue) with a selected dominantwavelength and at least one photoluminescence material that is operableto convert, through a process of photoluminescence, at least a portionof the excitation light to light of a different wavelength. The emissionproduct of the arrangement/device comprises the combined light generatedby the source and the photoluminescence material and is typicallyconfigured to appear white in color.

In this specification a “lighting arrangement” refers to a systemcomprising an LED-based light source and a photoluminescence wavelengthconversion component and as such includes lamps, down lights, spotlights, bulbs etc. In contrast a “light emitting device” refers to asystem in which the photoluminescence material in incorporated with theLED-based light source typically as a part of the packaging housing theLEDs.

In accordance with embodiments of the invention an LED-based lightsource for generating excitation light with the selected dominantwavelength comprises a plurality of LEDs that are selected from at leasttwo different wavelength bins. The number of LEDs and the wavelengthbins are selected such that the emission product of the source compriseslight having a dominant wavelength corresponding to the selectedwavelength. A particular advantage of the invention is that by combiningLEDs from different wavelength bins in a single package, the source cangenerate excitation light having a dominant wavelength that varies by asmaller amount between nominally the same sources compared with sourcescomposed of LEDs from a single wavelength bin. Moreover it is found thatthe variation in the dominant wavelength of such sources can besignificantly smaller than the wavelength variation within a singlewavelength bin. Furthermore the invention enables the use of lessexpensive LEDs from broader wavelength bins and/or LEDs from a number ofwavelength bins. Initial results indicate that using a light sourcecomposed of LEDs from at least two different wavelength bins enableslighting arrangements/devices to be constructed that generate light witha consistency of two MacAdam ellipses for a given photoluminescencematerial/photoluminescence wavelength conversion component. Whilst lightsources in accordance with embodiments of the invention find particularapplication in lighting arrangements that use a wavelength conversioncomponent in which the photoluminescence material(s) is/are locatedremotely to the source, the sources further provide benefits for lightemitting devices in which the photoluminescence material(s) areincorporated in the source package.

According to an embodiment of the invention a light source forgenerating light having a selected dominant wavelength comprises: apackage housing a plurality of LEDs comprising at least one first LEDfrom a first wavelength bin and at least one second LED from a secondwavelength bin, wherein the first wavelength bin comprises LEDs having adominant wavelength that is within a first wavelength range and thesecond wavelength bin comprises LEDs having a dominant wavelength thatis within a second wavelength range. The first wavelength bin cancomprise LEDs having a dominant wavelength that is shorter than theselected dominant wavelength and the second wavelength bin can compriseLEDs having a dominant wavelength that includes or is longer than theselected dominant wavelength.

The first and second LEDs can be selected from wavelength bins havingthe same peak luminous flux range. To enable each of the LEDs to beoperated from a single power source the first and second LEDs arepreferably selected from wavelength bins having the forward drivevoltage range.

The wavelength bin and number of LEDs from each bin are selected suchthat in operation the dominant wavelength of the combined light emittedby the light source substantially corresponds to the selected dominantwavelength. The inventors have discovered that by combining LEDs fromdifferent wavelength bins, this enables a light source to be constructedhaving an emission product whose dominant wavelength is within afraction, about one tenth, of the wavelength bin range of the selecteddominant wavelength. For example where the light source is configured togenerate blue light having a dominant wavelength in a range 450 nm to480 nm the wavelength range of each wavelength bin is can be 2.5 nm. Ithas been found that such a wavelength range enables a light source to beconstructed in accordance with the invention that has an emissionproduct whose dominant wavelength is within about ±0.2 nm of theselected dominant wavelength. Such an excitation source enables lightingarrangements/devices to be constructed that generate white light with aconsistency of two or less MacAdam ellipses.

In one embodiment the light source further comprises at least one thirdLED from a third wavelength bin in which the third wavelength bincomprises LEDs having a dominant wavelength that is within a thirdwavelength range and wherein the at least one LED is housed in thepackage. The third wavelength bin can comprise LEDs having a dominantwavelength that i) includes the selected dominant wavelength, ii) isshorter than the selected dominant wavelength or iii) is longer than theselected dominant wavelength.

To increase the CRI (Color Rendering Index) of generated light, thesource can further comprise at least one red LED that is operable togenerate red light of wavelength in a range 450 nm to 480 nm. The atleast one red LED can be housed in the package together with the blueLEDs. In one arrangement the at least one red LED comprises a pluralityof red LEDs comprising at least one first red LED from a firstwavelength bin and at least one second red LED from a second wavelengthbin, wherein the first wavelength bin comprises red LEDs having adominant wavelength that is within a first wavelength range and thesecond wavelength bin comprises red LEDs having a dominant wavelengththat is within a second wavelength range.

In some arrangements the LEDs are selected using the approximaterelationship:

$\lambda_{ds} \cong \frac{{n_{1} \cdot \lambda_{d\; 1} \cdot \Phi_{1}} + {n_{2} \cdot \lambda_{d\; 2} \cdot \Phi_{2}}}{{n_{1} \cdot \Phi_{1}} + {n_{2} \cdot \Phi_{2}}}$

where λ_(ds) is the selected dominant wavelength, n₁ is the number offirst LEDs, n₂ is the number of second LEDs, λ_(d1) is the dominantwavelength of the first LEDs, λ_(d2) is the dominant wavelength of thesecond LEDs, Φ₁ is the radiant flux of the first LEDs and Φ₂ is theradiant flux of the second LEDs.

The light source can further comprise at least one photoluminescencematerial, typically a phosphor material, that is configured to convertat least a portion of the light generated by the source to light of adifferent wavelength and wherein the emission product of the sourcecomprises the combination of light generated by the first and secondLEDs and photoluminescence light generated by the photoluminescencematerial. In some devices the photoluminescence material(s) can be mixedwith a light transmissive binder and the mixture applied to the LEDs.Where the source additionally comprises one or more red LEDs thephotoluminescence material mixture can be applied to the blue LEDs only.The light transmissive binder can comprise a curable liquid polymer suchas for example a polymer resin, a monomer resin, an acrylic, an epoxy, asilicone or a fluorinated polymer. Alternatively the LEDs can beencapsulated in a light transmissive material and the photoluminescencematerial(s) provided in a wavelength conversion component that isseparate from the source. In such wavelength conversion components thephotoluminescence material(s) can be incorporated within the componentand homogeneously distributed throughout the volume of the component orprovided as one or more layers on the surface of the component.

The package for housing the LEDs can comprise a cavity for housing arespective one of the LEDs. Alternatively the package can comprise oneor more cavities for housing multiple LEDs.

According to an embodiment of the invention a lighting arrangementcomprises: at least one light source for generating light having aselected dominant wavelength comprising a plurality of LEDs comprisingat least one first LED from a first wavelength bin in which the firstwavelength bin comprises LEDs having a dominant wavelength that iswithin a first wavelength range; at least one second LED from a secondwavelength bin in which the second wavelength bin comprises LEDs havinga dominant wavelength that is within a second wavelength range; and apackage housing the LEDs; and a wavelength conversion component locatedremotely to the at least one source and operable to convert at least aportion of the light generated by the at least one source to light of adifferent wavelength, wherein the emission product of the arrangementcomprises the combination of light generated by the at least one sourceand the wavelength conversion component; and wherein the wavelengthconversion component comprises a light transmissive substrate and atleast one photoluminescence material.

In one embodiment the photoluminescence material(s) is/are incorporatedin the light transmissive substrate and is/are preferably homogeneouslydistributed throughout the volume of the substrate. Alternatively and/orin addition the photoluminescence material can be provided as a layer ona surface of the light transmissive substrate. Preferably the wavelengthconversion layer comprises a mixture of the photoluminescence materialand a light transmissive binder. The light transmissive binder cancomprise a curable liquid polymer such as a polymer resin, a monomerresin, an acrylic, an epoxy, a silicone or a fluorinated polymer. Thephosphor layer can be deposited by screen printing, slot die coating,spin coating, roller coating, drawdown coating or doctor blading (i.e.using a flexible blade or squeegee to draw the material over thesurface).

The light transmissive substrate can comprise a plastics material suchas a polycarbonate or an acrylic or a glass.

The wavelength conversion component can be positioned on the at leastone light source or in a spaced relationship. When the component is in aspaced relationship it can be separated from the at least one source bya distance of at least 1 cm to reduce the transfer of heat and to reducethe likelihood of thermal degradation of the photoluminescencematerial(s).

According to an embodiment of the invention a light source forgenerating light having a selected dominant wavelength comprises apackage housing a plurality of blue LEDs, wherein the blue LEDs areselected from at least two different wavelength bins in which eachwavelength bin comprises LEDs that are operable to generate blue lighthaving a dominant wavelength that is in a respective wavelength range.The light source can further comprise one or more of red LEDs that areoperable to generate red light and which are housed in the package. Aswith the blue LEDs the red LED can comprise a plurality of red LEDs thatare selected from at least two different wavelength bins in which eachwavelength bin comprises red LEDs that are operable to generate redlight having a dominant wavelength that is in a respective wavelengthrange.

Whilst the invention arose in relation to white light emittingarrangements/devices the invention also finds utility inarrangements/devices that are operable to generate light of colors otherthan white.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention is better understood LED-basedlighting arrangements, light emitting devices and light sources inaccordance with embodiments of the invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 shows schematic partial cutaway plan and sectional views of anLED-based lighting arrangement in accordance with an embodiment of theinvention;

FIG. 2 shows schematic perspective and sectional views of an LED-basedlight bulb in accordance with an embodiment of the invention;

FIG. 3 shows schematic plan and sectional views of an LED-based lightsource in accordance with an embodiment of the invention for use in thearrangements of FIGS. 1 and 2;

FIG. 4 shows schematic plan and sectional views of an LED-based lightsource in accordance with an embodiment of the invention for use in thearrangements of FIGS. 1 and 2;

FIG. 5 is a schematic of radiant flux versus wavelength for an LED-basedlight source in accordance with the invention;

FIG. 6 shows schematic plan and sectional views of an LED-based lightsource in accordance with an embodiment of the invention for use in thearrangements of FIGS. 1 and 2;

FIG. 7 shows schematic plan and sectional views of an LED-based lightsource in accordance with an embodiment of the invention for use in thearrangements of FIGS. 1 and 2;

FIG. 8 shows schematic plan and sectional views of an LED-based lightemitting device in accordance with an embodiment of the invention; and

FIG. 9 shows schematic plan and sectional views of an LED-based lightemitting device in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this patent specification like reference numerals are used todenote like parts.

LED-Based Lighting Arrangements (Lamps)

An LED-based lighting arrangement (lamp) 10 in accordance with anembodiment of the invention will now be described with reference to FIG.1 which shows schematic partial cutaway plan and sectional views of thelamp. The lamp 10 is configured to generate warm white light with a CCT(Correlated Color Temperature) of approximately 3000K and a luminousflux of approximately 600 lumens. The lamp can be used, for example, forunder cabinet lighting or incorporated into other lighting fixtures suchas a down light, spot light, troffer etc.

The lamp 10 comprises a hollow cylindrical body 12 composed of acircular disc-shaped base 14, a hollow cylindrical wall portion 16 and adetachable annular top 18. To aid in the dissipation of heat the base 14is preferably fabricated from aluminum, an alloy of aluminum or anymaterial with a high thermal conductivity (preferably ≧200 Wm⁻¹K⁻¹) suchas for example copper, a magnesium alloy or a metal loaded plasticsmaterial. For low cost production the wall 16 and top 18 are preferablyfabricated from a thermoplastics material such as HDPP (High DensityPolypropylene), nylon or PMA (polymethyl acrylate). Alternatively theycan be fabricated from a thermally conductive material such as aluminumor an aluminum alloy. As indicated in FIG. 1 the base 14 can be attachedto the wall portion 16 by screws or bolts 20 or by other fasteners orusing an adhesive. As further shown in FIG. 1 the top 18 can bedetachably mounted to the wall portion 16 using a bayonet-type mount inwhich radially extending tabs 22 engage in a corresponding annulargroove in the top 18. The top 18 can be mounted and dismounted byrotating the top relative to the body. In other arrangements the top canbe affixed to the wall portion using an adhesive or other fasteningmeans.

The lamp 10 further comprises a plurality (four in the exampleillustrated) of blue light emitting LED-based light sources 24 that aremounted in thermal communication with a circular-shaped MCPCB (metalcore printed circuit board) 26. The light sources 24 are described indetail below and are operable to generate blue excitation light 28(λ_(dS)) with a selected dominant wavelength that is typically in arange 450 nm to 480 nm. As is known an MCPCB comprises a layeredstructure composed of a metal core base, typically aluminum, a thermallyconductive/electrically insulating dielectric layer and a copper circuitlayer for electrically connecting electrical components in a desiredcircuit configuration. The metal core base of the MCPCB 26 is mounted inthermal communication with the base 14 with the aid of a thermallyconductive compound such as for example an adhesive containing astandard heat sink compound containing beryllium oxide or aluminumnitride. As shown in FIG. 1 the MCPCB can be attached to the base usingscrews or bolts 30.

To maximize the emission of light, the lamp 10 can further compriselight reflective surfaces 32, 34 that respectively cover the face of theMCPCB 26 and the inner curved surface of the top 18. Typically the lightreflective surfaces 32, 34 can comprise a highly light reflective sheetmaterial such as WhiteOptics™ “White 97” (a high-density polyethylenefiber-based composite film) from A.L.P. lighting Components, Inc ofNiles, Ill., USA. As indicated in FIG. 1 a circular disc 32 of thematerial can be used to cover the face of the MCPCB with apertures foreach source and a strip of the light reflective material configured as acylindrical sleeve 34 that is inserted in the housing and is configuredto cover the inner surface of the housing wall portion 16.

The lamp 10 further comprises a photoluminescence wavelength conversioncomponent 36 that is operable to absorb a proportion of the blueexcitation light 28 (λ_(ds)) generated by the sources 24 and convert itto light 38 of a different wavelength (λ_(p)) by a process ofphotoluminescence 36. The emission product 40 of the lamp 10 comprisesthe combined light 28, 38 of wavelengths λ_(ds), λ_(p) generated by thelight sources 24 and the photoluminescence wavelength conversioncomponent 36. The wavelength conversion component 36 (more particularlythe photoluminescence material) is positioned remotely to the lightsources 24 and is spatially separated from the light sources by an airgap of distance d that is typically at least 1 cm. In this patentspecification “remotely” and “remote” means in a spaced or separatedrelationship. The separation can comprise an air gap as illustrated inFIG. 1 or a light transmissive medium. The wavelength conversioncomponent 36 is configured to completely cover the housing 12 openingsuch that all light emitted by the device passes through the component36. As shown the wavelength conversion component 36 can be detachablymounted to the top of the wall portion 16 using the top 18 enabling thecomponent and emission color of the lamp to be readily changed. In otherarrangements the wavelength conversion component can be affixed to thehousing.

The wavelength conversion component comprises a light transmissivesubstrate 42 and a wavelength conversion layer 44 containing one or morephotoluminescence materials. Typically the photoluminescence material(s)comprises a phosphor material though they can comprise otherphotoluminescence materials such as a quantum dot material or acombination thereof. As shown in FIG. 1 the wavelength conversioncomponent 36 is configured such that in operation the wavelengthconversion layer 44 faces the light sources 24. Additionally (not shown)the wavelength conversion component can comprise a light diffusing layercomprising a uniform thickness layer of light diffusive particles suchas titanium dioxide (TiO₂), barium sulfate (BaSO₄), magnesium oxide(MgO), silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃). The lightdiffusive material can not only improve the color uniformity ofgenerated light but can additionally improve the off-state whiteappearance of the wavelength conversion component.

The light transmissive substrate 42 can be any material that issubstantially transmissive to light over a wavelength range 380 nm to740 nm and can comprise a light transmissive polymer such as apolycarbonate or acrylic or a glass such as a borosilicate glass. Forexample in FIG. 1 the substrate 42 comprises a planar circular disc ofdiameter φ=62 mm and thickness t₁ which is typically 0.5 mm to 3 mm. Inother embodiments the substrate can comprise other geometries such asbeing convex or concave in form such as for example being dome-shaped,dish-shaped or cylindrical.

The wavelength conversion layer 44 can comprise a uniform thicknesslayer composed of the photoluminescence material(s) and a lighttransmissive binder material. The binder can comprise a curable liquidpolymer such as a polymer resin, a monomer resin, an acrylic, an epoxy(polyepoxide), a silicone or a fluorinated polymer. It is important thatthe binder material is, in its cured state, substantially transmissiveto all wavelengths of light generated by the phosphor material(s) andthe light sources 24 and preferably has a transmittance of at least 0.9over the visible spectrum (380 nm to 800 nm). The binder material ispreferably U.V. curable though it can be thermally curable, solventbased or a combination thereof. U.V. or thermally curable binders can bepreferable because, unlike solvent-based materials, they do not “outgas”during polymerization.

Where the photoluminescence material comprises a phosphor material,which is in powder form, this is thoroughly mixed in known proportionswith the liquid binder material to form a suspension and the resultingphosphor composition, “phosphor ink”, deposited onto the lighttransmissive substrate. The wavelength conversion layer 44 is preferablydeposited by screen printing though other deposition techniques such asslot die coating, spin coating, roller coating, drawdown coating ordoctor blading can be used. The color of the emission product producedby the wavelength conversion component 36 will depend on the phosphormaterial composition and the quantity of phosphor material per unit areain the wavelength conversion layer 44. It will be appreciated that thequantity of phosphor material per unit area is dependent on thethickness of the wavelength conversion layer 44 and the weight loadingof phosphor material to binder in the phosphor ink. In applications inwhich the emission product is white or in applications in which theemission product has a high saturation color (i.e. the emission productcomprises substantially all photoluminescence generated light) thequantity of phosphor material per unit area in the wavelength conversionlayer 44 will typically be between 10 mg·cm⁻² and 40 mg·cm⁻². To enableprinting of the wavelength conversion layer 44 in a minimum number ofprint passes the phosphor ink preferably has as high a solids loading ofphosphor material to binder material as possible and preferably has aweight loading of phosphor material to binder in a range 40% to 75%. Thephosphor material comprises particles with an average particle size of10 μm to 20 μm and typically of order 15 μm. In alternative arrangements(not shown) the phosphor material can be incorporated in the lighttransmissive substrate and homogeneously distributed throughout thevolume of the substrate.

In general lighting applications the emission product 40 will typicallybe white light and the photoluminescence material can comprise one ormore blue light excitable phosphor materials that emit green (510 nm to550 nm), yellow-green (550 nm to 570 nm), yellow (570 nm to 590 nm),orange (590 nm to 630 nm) or red (630 nm to 740 nm) light. The thicknessof the wavelength conversion layer 44, phosphor material composition andthe density (weight loading) of phosphor material per unit area willdetermine the color of light emitted by the lamp 10.

The photoluminescence material can comprise an inorganic or organicphosphor such as for example silicate-based phosphor of a generalcomposition A₃Si(O,D)₅ or A₂Si(O,D)₄ in which Si is silicon, O isoxygen, A comprises strontium (Sr), barium (Ba), magnesium (Mg) orcalcium (Ca) and D comprises chlorine (Cl), fluorine (F), nitrogen (N)or sulfur (S). Examples of silicate-based phosphors are disclosed inU.S. Pat. No. 7,575,697 B2 “Silicate-based green phosphors” (assigned toIntematix Corp.), U.S. Pat. No. 7,601,276 B2 “Two phase silicate-basedyellow phosphors” (assigned to Intematix Corp.), U.S. Pat. No. 7,655,156B2 “Silicate-based orange phosphors” (assigned to Intematix Corp.) andU.S. Pat. No. 7,311,858 B2 “Silicate-based yellow-green phosphors”(assigned to Intematix Corp.). The phosphor can also comprise analuminate-based material such as is taught in our co-pending patentapplication US2006/0158090 A1 “Novel aluminate-based green phosphors”and patent U.S. Pat. No. 7,390,437 B2 “Aluminate-based blue phosphors”(assigned to Intematix Corp.), an aluminum-silicate phosphor as taughtin co-pending application US2008/0111472 A1 “Aluminum-silicateorange-red phosphor” or a nitride-based red phosphor material such as istaught in our co-pending United States patent application US2009/0283721A1 “Nitride-based red phosphors” and International patent applicationWO2010/074963 A1 “Nitride-based red-emitting in RGB (red-green-blue)lighting systems”. It will be appreciated that the phosphor material isnot limited to the examples described and can comprise any phosphormaterial including nitride and/or sulfate phosphor materials,oxy-nitrides and oxy-sulfate phosphors or garnet materials (YAG).

FIG. 2 shows perspective and cross sectional views of a LED-based lightbulb (lamp) 10 in accordance with an embodiment of the invention. Thelamp 10 comprises a generally conical shaped thermally conductive body46 that includes a plurality of latitudinal heat radiating fins (veins)48 circumferentially spaced around the outer curved surface of the body46 to aid in the dissipation of heat. The lamp 10 further comprises aconnector cap (Edison screw lamp base) 50 enabling the lamp to bedirectly connected to a power supply using a standard electricallighting screw socket. The connector cap 50 is mounted to the truncatedapex of the body 46. The lamp 10 further comprises one or more LED-basedlight sources 24 mounted in thermal communication with the base of thebody 46. In order to generate white light the lamp 10 further comprisesa photoluminescence wavelength conversion component 36 mounted to thebase of the body and configured to enclose the sources 24. As indicatedin FIG. 2 the wavelength conversion component 36 can be a generally domeshaped shell and includes one or more photoluminescence materials toprovide wavelength conversion of blue light generated by the sources 24.The photoluminescence material can be incorporated in and homogeneouslydistributed throughout the volume of the component or provided as one orlayers on the inner or outer surfaces of the component. For ease offabrication the photoluminescence wavelength conversion component can beinjection molded. For aesthetic considerations the lamp can furthercomprise a light transmissive envelope 52 which encloses the wavelengthconversion component 36.

LED-Based Light Source

In accordance with embodiments of the invention the LED-based lightsources 24 can comprise a package or housing 54 that houses a pluralityof first and second LED dies 56, 58. In the example shown in FIG. 3 thepackage comprises a low temperature co-fired ceramic (LTCC) having anarray of circular recesses or cavities 60 in which each recess isconfigured to house a respective one of the LED dies 56, 58. The packagecan as shown comprise a square array of twelve cavities 60 (three rowsby four columns) for housing six first LED dies 56 and six second LEDdies 58. To provide protection of the LED dies 56, 58 and bond wireseach recess 60 is filled with a light transmissive encapsulation 62 suchas a silicone or epoxy. Each of the LED dies preferably comprises a bluelight emitting GaN-based (gallium nitride-based) LED die. In accordancewith the invention each of the first blue LED dies 56 are from a firstwavelength bin that comprises LED dies having a dominant wavelengthλ_(d1) that is within a first wavelength range whilst each of the secondblue LED dies 58 are from a second different wavelength bin thatcomprises LED dies having a dominant wavelength λ_(d2) that is within asecond different wavelength range. As is further described the numberand radiant flux of the LED dies from the different wavelength bins andthe choice of wavelength bins are selected such that the light source 24generates light 28 having a dominant wavelength corresponding to theselected wavelength λ_(ds).

An example of an LED-based light source 24 in accordance with a furtherembodiment of the invention is shown in FIG. 4. In this embodiment thepackage 54 has a single circular recess 60 that houses the first andsecond blue LED dies 56, 58. In the example shown in FIG. 4 the packagehouses a hexagonal array of twenty three LED dies comprising elevenfirst blue LED dies 46 and twelve second blue LED dies 58. The cavityrecess can be filled with a light transmissive encapsulation 62 toprotect the LED dies and wire bonds.

As is known LED dies are commonly classified (grouped) by a processknown as binning Typically the LED dies are binned according to thedominant wavelength λ_(d) of light they generate and the radiant flux Φ(defined in watts) of light they generate for a selected forward voltage(V_(F)).

TABLE 1 shows an example of a binning scheme for blue LED dies. In theexample there are thirty two bins comprising four bins for each of eightwavelength range bins. For example bin 9 comprises LED dies thatgenerate light having a dominant wavelength λ_(d) in a range 450.0 nm to452.5 nm and a radiant flux Φ₁ for a forward voltage V_(F1) whilst bin21 comprises LED dies that generate light having a dominant wavelengthλ_(d) in a range 457.5 nm to 460.0 nm) and a radiant flux Φ₁ for aforward voltage V_(F1).

TABLE 1 wavelength Dominant LED Bin bin wavelength Radiant Flux Φ₁Radiant Flux Φ₂ λ_(d)(nm) range (nm) for V_(F1) ,V_(F2) for V_(F1),V_(F2) 445.0 445.0 to 447.5 1, 2 3, 4 447.5 447.5 to 450.0 5, 6 7, 8450.0 450.0 to 452.5  9, 10 11, 12 452.5 452.5 to 455.0 13, 14 15, 16455.0 455.0 to 457.5 17, 18 19, 20 457.5 457.5 to 460.0 21, 22 23, 24460.0 460.0 to 462.5 25, 26 27, 28 462.5 462.5 to 465.0 29, 30 31, 32

The method of selecting the first and second LED dies 56, 58 to ensurethat the source 24 generates light with the selected emission wavelengthλ_(ds) is now described with reference to FIG. 5 which shows plots ofradiant flux versus wavelength for i) the first LED dies, ii) the secondLED dies and iii) the combined output for an LED-based light source inaccordance with an embodiment of the invention. As indicated by solidline 64 the first LED dies 56 generate blue light with a dominantwavelength λ_(d1) and a peak luminous flux Φ₁. The first LED dies 56 areselected from a first wavelength bin whose dominant wavelength range isshorter than the selected wavelength λ_(ds). As indicated by dashed line66 the second LED dies 58 generate blue light with a dominant wavelengthλ_(d2) and a peak luminous flux Φ₂. The second LED dies 58 are selectedfrom a second wavelength bin whose dominant wavelength range is longerthan the selected dominant wavelength λ_(ds). Dotted line 68 shows thecombined output 28 from the source of the selected dominant wavelengthλ_(ds) and peak luminous flux Φ_(s).

For a source comprising two LED dies in which the first LED die is froma first wavelength bin with a dominant wavelength λ_(d1) and peakradiant flux Φ₁ and the second LED die is from a second bin with adominant wavelength λ_(d2) and radiant flux Φ₂ the combined lightemitted by the source has a dominant wavelength λ_(ds) that isapproximately given by the relation:

$\lambda_{ds} \cong \frac{{\lambda_{d\; 1} \cdot \Phi_{1}} + {\lambda_{d\; 2} \cdot \Phi_{2}}}{\Phi_{1} + \Phi_{2}}$

For a source comprising n₁ first LED dies and n₂ second LED dies thesource has a dominant wavelength λ_(ds) that is approximately given bythe relationship:

$\lambda_{ds} \cong \frac{{n_{1} \cdot \lambda_{d\; 1} \cdot \Phi_{1}} + {n_{2} \cdot \lambda_{d\; 2} \cdot \Phi_{2}}}{{n_{1} \cdot \Phi_{1}} + {n_{2} \cdot \Phi_{2}}}$

If the first and second LED dies are selected from bins having the samepeak luminous flux (i.e. Φ₁=Φ₂) the relationship reduces to:

$\lambda_{ds} \cong \frac{{n_{1} \cdot \lambda_{d\; 1}} + {n_{2} \cdot \lambda_{d\; 2}}}{n_{1} + n_{2}}$

It is to be noted that whilst the forward drive voltage V_(F) of the LEDdies does not affect the emission wavelength of the source it canhowever be preferable to select the LED dies from bins that have thesame forward drive voltage to enable each of the LED dies to be operatedfrom a single power source. Alternatively the LED dies can be grouped byforward drive voltage enabling each group to be driven using a commonpower source.

TABLE 2 tabulates examples of LED bins for the first and second LED dies56, 58 bins for a source configured to generate blue light with aselected dominant wavelength λ_(ds) of 455 nm. As can be seen from thetable whilst the LED bins have a dominant wavelength range of 2.5 nm(±1.25 nm) initial tests indicate that sources in accordance with theinvention can unexpectedly generate light with a selected dominantwavelength that varies over a range of 0.4 nm (±0.2 nm). When suchsources 24 are used within the lamp 10 of FIG. 1 it is found that thelamp can generate white light that on a chromaticity diagram is withintwo MacAdam ellipses of the selected light color. For comparison anidentical lamp 10 having light sources 24 comprising a single type ofLED die generates light that is typically within about five MacAdamellipses of the selected color. It will be appreciated that the lightsources of the invention enable the construction of lamps that produce amore accurate color of light. Alternatively the invention enables lessexpensive LED dies from wider wavelength bins to construct lamps thatproduce light of a consistent color.

TABLE 2 1^(st) wavelength No. of 1^(st) 2^(nd) wavelength No. of 2^(nd)Dominant bin LED bin LED wavelength of λ_(d1) (nm) dies n₁ λ_(d2) (nm)dies n₂ source λ_(ds) 445.0 6 462.5 6 455.0 ± 0.2  447.5 6 460.0 6 455.0± 0.2  450.0 6 457.5 6 455.0 ± 0.2  452.5 6 455.0 6 455.0 ± 0.2 

It is envisioned in further embodiments to increase accuracy of theselected dominant wavelength of the source by increasing the totalnumber of LED dies 56, 58. For example and as indicated in TABLE 3 for asource comprising a total of thirty six LED dies (eighteen first LEDdies 56 and eighteen second LED dies 58) initial tests suggest that thesource can generate light having a selected wavelength that is within0.2 nm (±0.1 nm), that is about a tenth of the bin wavelength range.

TABLE 3 1^(st) wavelength No. of 1^(st) 2^(nd) wavelength No. of 2^(nd)Dominant bin LED bin LED wavelength of λ_(d1) (nm) dies n₁ λ_(d2) (nm)dies n₁ source λ_(ds) 445.0 18 462.5 18 455.0 ± 0.1  447.5 18 460.0 18455.0 ± 0.1  450.0 18 457.5 18 455.0 ± 0.1  452.5 18 455.0 18 455.0 ±0.1 

FIG. 6 shows a LED-based light source that comprises LED dies from threewavelength bins. In FIG. 6 the light source comprises a total of nineLED dies comprising three LED dies 56 from a first wavelength bin, threeLED dies 58 from a second wavelength bin and three LED dies 70 from athird wavelength bin. For a source comprising n₁ first LED dies from afirst wavelength bin with a dominant wavelength λ_(d1) and peak radiantflux Φ₁, n₂ second LED dies from a second bin with a dominant wavelengthλ_(d2) and radiant flux Φ₂ and n₃ third LED dies from a third bin with adominant wavelength λ_(d3) and radiant flux Φ₃ the combined lightemitted by the source has a dominant wavelength λ_(ds) that isapproximately given by the relationship:

${\lambda_{d\; s} \cong {\frac{{n_{1} \cdot \lambda_{d\; 1} \cdot \Phi_{1\;}} + {n_{2} \cdot \lambda_{d\; 2} \cdot \Phi_{2}} + {n_{3} \cdot \lambda_{d\; 3} \cdot \Phi_{3}}}{{n_{1} \cdot \Phi_{1}} + {n_{2} \cdot \Phi_{2}} + {n_{3} \cdot \Phi_{3}}}.}}\;$

It is envisioned in further LED-based sources for generating blue lightwith a selected dominant wavelength it is envisioned to house blue LEDdies from four or more wavelength bins.

Moreover whilst in the foregoing embodiments the source is described ascomprising equal numbers of LED dies from different bins it will beappreciated that the source can comprise differing numbers of LED dies.

In applications where it is required for the lamp 10 to generate lightwith a high CRI (Color Rendering Index), that is a CRI>90, it isenvisioned for the source 24 to additionally comprise one or more redemitting LED dies that are operable to generate red light of wavelengthin a range 630 nm to 740 nm. FIG. 7 shows such a light source 24 thatcomprises five blue LED dies 56 from a first wavelength bin, five blueLED dies 58 from a second wavelength bin and two red LED dies 72. Aswith the blue LED dies the source can comprise a plurality of red LEDdies that are selected from different wavelength bins such that theircombined output is red light of a selected dominant wavelength.

LED-Based Light Emitting Devices

Whilst the LED-based light sources 24 of the invention find particularapplication in lighting arrangements and lamps that comprise a separatewavelength conversion component 36 the invention also finds applicationin light emitting devices in which the photoluminescence material isincorporated in the device as opposed to being provided in a separatewavelength conversion component. Examples of such devices are shown inFIGS. 8 and 9.

In the example of FIG. 8 the light emitting device 74 comprises apackage 54 having a single circular recess 60 that houses the first andsecond blue LED dies 56, 58. As indicated the LED dies are configured asa hexagonal array of twenty three LED dies comprising eleven first blueLED dies 56 and twelve second blue LED dies 58. The recess 60 is filledwith a light transmissive encapsulation 76 that includes one or morephotoluminescence materials. It will be appreciated that the device ofFIG. 8 comprises the light source 24 of FIG. 3 with thephotoluminescence material incorporated in the device in the form of anencapsulation 76.

In the example show in FIG. 9 the device 74 comprises a package 54having an array of circular recesses 60 in which each recess isconfigured to house a respective one of the LED dies 56, 58. The packagecan as shown comprise a square array of twelve cavities 60 (three rowsby four columns) for housing six first LED dies 56 and six second LEDdies 58. Each of the recesses 60 is filled with a light transmissiveencapsulation 76 that includes one or more photoluminescence materials.It will be appreciated that the device of FIG. 9 comprises the lightsource 24 of FIG. 2 with the photoluminescence material incorporated inthe device in the form of an encapsulation 76.

It will be appreciated that the invention is not limited to theexemplary embodiments described and that variations can be made withinthe scope of the invention. For example the number of LED dies, numberof LED bins and packaging arrangements can be varied without departingfrom the inventive concepts of the invention.

1. An LED-based light source for generating light having a selecteddominant wavelength comprising: a package housing a plurality of LEDscomprising at least one first LED from a first wavelength bin and atleast one second LED from a second wavelength bin, wherein the firstwavelength bin comprises LEDs having a dominant wavelength that iswithin a first wavelength range and the second wavelength bin comprisesLEDs having a dominant wavelength that is within a second wavelengthrange.
 2. The light source of claim 1, wherein the first wavelength bincomprises LEDs having a dominant wavelength that is shorter than theselected dominant wavelength.
 3. The light source of claim 1, whereinthe second wavelength bin comprises LEDs having a dominant wavelengththat is longer than the selected dominant wavelength.
 4. The lightsource of claim 1, wherein the wavelength bin and number of LEDs fromeach wavelength bin are selected such that in operation the dominantwavelength of the combined light emitted by the source is the selecteddominant wavelength.
 5. The light source of claim 1, wherein the firstand second LEDs are from wavelength bins selected from the groupconsisting of: wavelength bins having substantially the same peakluminous flux range; wavelength bins having substantially the sameforward drive voltage range and combinations thereof.
 6. The lightsource of claim 1, and further comprising at least one third LED from athird wavelength bin in which the third wavelength bin comprises LEDshaving a dominant wavelength that is within a third wavelength range andwherein the at least one LED of the third bin are housed in the package.7. The light source of claim 1, wherein the first and second LEDs areoperable to generate blue light of wavelength in a range 450 nm to 480nm.
 8. The light source of claim 7, and further comprising at least onered LED that is operable to generate red light of wavelength in a range450 nm to 480 nm and wherein the at least one red LED is housed in thepackage.
 9. The light source of claim 8, wherein the at least one redLED comprises a plurality of red LEDs comprising at least one first redLED from a first wavelength bin and at least one second red LED from asecond wavelength bin, wherein the first wavelength bin comprises redLEDs having a dominant wavelength that is within a first wavelengthrange and the second wavelength bin comprises LEDs having a dominantwavelength that is within a second wavelength range.
 10. The lightsource of claim 1, wherein the LEDs are selected using the approximaterelationship:$\lambda_{ds} \cong \frac{{n_{1} \cdot \lambda_{d\; 1} \cdot \Phi_{1}} + {n_{2} \cdot \lambda_{d\; 2} \cdot \Phi_{2}}}{{n_{1} \cdot \Phi_{1}} + {n_{2} \cdot \Phi_{2}}}$where λ_(sd) is the selected dominant wavelength, n₁ is the number offirst LEDs, n₂ is the number of second LEDs, λ_(d1) is the dominantwavelength of the first LEDs, λ_(d2) is the dominant wavelength of thesecond LEDs, Φ₁ is the radiant flux of the first LEDs and Φ₂ is theradiant flux of the second LEDs.
 11. The light source of claim 1,wherein the package comprises a cavity for housing a respective one ofthe LEDs.
 12. A lighting arrangement comprising: at least one lightsource for generating light having a selected dominant wavelengthcomprising a plurality LEDs comprising at least one first LED from afirst wavelength bin and at least one second LED from a secondwavelength bin, wherein the first wavelength bin comprises LEDs having adominant wavelength that is within a first wavelength range and thesecond wavelength bin comprises LEDs having a dominant wavelength thatis within a second wavelength range; and a wavelength conversioncomponent located remotely to the at least one source and operable toconvert at least a portion of the light generated by the at least onesource to light of a different wavelength, wherein the emission productof the arrangement comprises the combination of light generated by thesource and the wavelength conversion component; and wherein thewavelength conversion component comprises a light transmissive substrateand at least one photoluminescence material.
 13. The lightingarrangement of claim 12, wherein the at least one photoluminescencematerial is selected from the group consisting of: being incorporated inthe light transmissive substrate; being provided as a layer on at leasta part of a surface of the light transmissive substrate; andcombinations thereof.
 14. The lighting arrangement of claim 13, whereinthe wavelength conversion component is separated from the at least onesource by a distance of at least 1 cm.
 15. The lighting arrangement ofclaim 12, wherein the first wavelength bin comprises LEDs having adominant wavelength that is shorter than the selected dominantwavelength.
 16. The lighting arrangement of claim 12, wherein the secondwavelength bin comprises LEDs having a dominant wavelength that islonger than the selected dominant wavelength.
 17. The lightingarrangement of claim 12, wherein the first and second LEDs are selectedfrom the group consisting of: wavelength bins having substantially thesame peak luminous flux range; wavelength bins having substantially thesame forward drive voltage range; and combinations thereof.
 18. Thelighting arrangement of claim 12, wherein the wavelength bin and thenumber of LEDs from each wavelength bin are selected such that inoperation the dominant wavelength of the combined light emitted by thearrangement is the selected dominant wavelength.
 19. The lightingarrangement of claim 12, and further comprising at least one of thirdLED from a third wavelength bin in which the third wavelength bincomprises LEDs having a dominant wavelength that is within a thirdwavelength range and wherein the at least one LED of the third bin ishoused in the package.
 20. The lighting arrangement of claim 12, whereinthe first and second LEDs are operable to generate blue light ofwavelength in a range 450 nm to 480 nm.
 21. The lighting arrangement ofclaim 20, and further comprising at least one red LED that is operableto generate red light of wavelength in a range 450 nm to 480 nm andwherein the at least one red LED is housed in the package.
 22. Thelighting arrangement of claim 21, wherein the at least one red LEDcomprises a plurality of red LEDs comprising at least one first red LEDfrom a first wavelength bin and at least one second red LED from asecond wavelength bin, wherein the first wavelength bin comprises redLEDs having a dominant wavelength that is within a first wavelengthrange and the second wavelength bin comprises red LEDs having a dominantwavelength that is within a second wavelength range.
 23. An LED-basedlight emitting device comprising: at least one light source forgenerating light having a selected dominant wavelength comprising aplurality LEDs comprising at least one first LED from a first wavelengthbin and at least one second LED from a second wavelength bin, whereinthe first wavelength bin comprises LEDs having a dominant wavelengththat is within a first wavelength range and the second wavelength bincomprises LEDs having a dominant wavelength that is within a secondwavelength range; and at least one photoluminescence material that isoperable to convert at least a portion of the light generated by the atleast one source to light of a different wavelength, wherein theemission product of the device comprises the combination of lightgenerated by the source and the at least one photoluminescence material.24. An LED-based light source for generating blue light having aselected dominant wavelength comprising a package housing a plurality ofblue LEDs, wherein the blue LEDs are selected from at least twodifferent wavelength bins in which each wavelength bin comprises LEDsthat are operable to generate blue light having a dominant wavelengththat is in a respective wavelength range.
 25. The light source of claim24, and further comprising at least one red LED that is operable togenerate red light and wherein the at least one red LED is housed in thepackage.